The present invention relates to a plain bearing having a sliding layer on the surface of a bearing alloy layer formed of a copper-based or aluminum-based alloy.
Bearings for automobile engine have been those obtained by bonding a copper-based bearing alloy or an aluminum-based bearing alloy to the surface of a back metal made of a steel plate. Attempts were made to improve such bearings in wear resistance, anti-seizure property and initial conformability by forming a sliding layer as coating layer on the surface of a bearing alloy layer by incorporating a solid lubricant and the like into a thermosetting resin such as a polyamide-imide resin (a PAI resin), a polyimide resin (a PI resin) or an epoxy resin (an EP resin) as disclosed, for example, in JP-A-4-83914 (patent document 1) and JP-A-9-79262 (patent document 2). In addition, an attempt was made to improve the early wear resistance while maintaining the conformability, by forming a protective layer out of a solid lubricant and a binder composed of a thermoplastic resin and a thermosetting resin which are soluble in a polar solvent, as disclosed in JP-A-2001-343022 (patent document 3).
There is the following phenomenon: during the use of the plain bearing, cavities (bubbles) are produced in lubricating oil, so that erosion is caused on the surface of the bearing. This phenomenon is such that since the cavities produced in the lubricating oil are disintegrated under a high pressure, energy at the time of the disintegration destroys the surface of the bearing in the manner of attack. As to a countermeasure against this phenomenon, the resistance to cavitation has been improved by increasing the strength of a material for a sliding layer. For example, in JP-A-2004-19758 (patent document 4), an attempt was made to improve the wear resistance, anti-seizure property and cavitation resistance by forming a sliding layer by incorporating a solid lubricant and the like into a polybenzimidazole resin (a PBI resin). Furthermore, in JP-A-2003-56566 (patent document 5), the sliding characteristics are improved by the use of a solid lubricant containing lead and a resin binder comprising at least one resin selected from a PAI resin, a PI resin, an EP resin and a PBI resin.
The inventions disclosed in the above-mentioned patent documents 1 to 5, however, do not sufficiently satisfy the demand for the bearing capability (anti-seizure property, initial conformability and cavitation resistance) of the plain bearing based on performance characteristics improvement and a load increase required with a recent increase in the power and revolution of internal combustion engines. For example, in the case of the invention disclosed in the above-mentioned patent document 5, the production of a resin binder by mixing of a PAI resin, a PI resin, an EP resin and a PBI resin is described. However, in such a simple polymer blend, the resins are not blended with one another and are merely dispersed as clusters. Therefore, the resin binder is disadvantageous in that since there is the nonuniformity of physical properties in a sliding surface, a sufficient anti-seizure property, in particular, a sufficient cavitation resistance cannot be attained.
The above-mentioned patent document 3 states “when a thermosetting resin and a thermoplastic resin are dissolved in a solvent, they are entirely mixed with each other as very fine units similar to molecules and the resulting binder has properties intermediate between those of the thermoplastic resin and those of the thermosetting resin.” The expression “entirely mixed with each other” means that the thermosetting resin and the thermoplastic resin are in the form of clusters though the clusters are very fine (micro-clusters). Therefore, the thermosetting resin and the thermoplastic resin are not in a blended state and the properties intermediate between those of the thermosetting resin and those of the thermoplastic resin are attained by the fine dispersion of the thermoplastic resin in the thermosetting resin. Such a binder is disadvantageous in that when it is used in the sliding layer of a plain bearing used under conditions under which cavitation phenomenon occurs, stress due to cavitation is centered at the boundary surface between the resin phases at which physical properties become discontinuous, so that the cavitation resistance is deteriorated.